The epoxy cresol novolac (ECN) resins have become important items of commerce with a myriad of uses based on their highly desirable physical properties after curing. These ECN resins, with a multiplicity of glycidyl ether groups in each molecule, lead to cured resins with high crosslink density and great rigidity. Such properties are particularly important in the electronics industry for circuit boards and encapsulation of electronic devices.
Aqueous sodium hydroxide solutions (50%) are used in the addition and dehydrochlorination steps in conventional ECN and EPN preparation processes. The water introduced by the solution contributes to the resulting products having low epoxy values. Conventional ECN resins have epoxy values in the range of 0.44-0.46 equivalents/100 gram resin, and conventional EPN resins have epoxy values in the range of 0.55-0.57 equivalents/100 grams resin. While such values characterize resins with excellent properties as mentioned above, resins with still higher epoxy values would open new markets for applications demanding even higher levels of performance. As smaller and more complex electronic devices are developed, the requirements for the performance of the resins used to protect them will continue to become more and more stringent.
Novolacs are prepared by the acid catalyzed reaction of a phenol, such as phenol, o-cresol, m-cresol, p-cresol or the like, with formaldehyde. Epoxy novolacs are formed by reaction of the novolac with epichlorohydrin first to form the corresponding chlorohydrin ether intermediate followed by dehydrochlorination to the epoxy novolac itself.
The patent literature describes a number of processes for the manufacture of glycidyl ethers which use catalysts for the chlorohydrin ether formation. The processes are, in some cases, very involved and their products do not have the desired low chlorine content. In addition the presence of residual catalyst or catalyst by-product in the resin may adversely affect product performance.
According to the process described in U.S. Pat. No. 3,336,342, polyhydric phenols are reacted with epihalogenohydrins in the presence of sulphonium salts, or compounds containing sulphur which can react with epihalogenohydrin to give sulphonium salts, to form the corresponding halogenohydrins from which, after removing the excess epihalogenohydrin, hydrogen halide is split off so as to arrive at the desired epoxide compounds. This process is very time consuming since the formation of the chlorohydrin ether requires at least 40 hours. Furthermore, the recovered excess epihalogenohydrin distilled off contains some dihalogenohydrin which must be worked up separately before being reused. For these reasons the process is very time-consuming, involved and uneconomical.
According to the process described in U.S. Pat. No. 3,372,142, not only carboxylic acids but also phenols are converted into the chlorohydrin compounds by means of excess epichlorohydrin in the presence of benzyltrimethylammonium chloride or anionic exchange resins and thereafter converted into the epoxide compounds with an aqueous solution of an alkali metal hydroxide which is saturated with an alkali metal carbonate. Here again it is found that the process is much too time-consuming for practical use since the formation of the chlorohydrin ether requires 25 hours, excluding the work-up of the chlorohydrin ether to give the epoxide compound which would require a further 10-15 hours; the kettle occupancy time would be unacceptable in practice.
A similar process is described in U.S. Pat. No. 2,943,096, according to which, again, polyhydric phenols and epichlorohydrin are converted into the chlorohydrin ether, in the presence of tetramethylammonium chloride or benzyltrimethylammonium chloride. This again requires 25 hours. The subsequent work-up of the batch proves to be very expensive since the excess epichlorohydrin, after being recovered by distillation, must be treated with sodium hydroxide solution to reduce its dichlorohydrin content before reuse. The isolated chlorohydrin ether is dissolved in a solvent mixture of toluene/ethanol and converted into the glycidyl ether by reaction with 18 weight % aqueous sodium hydroxide solution. Here again the individual process steps require a great deal of time so that this process cannot be regarded as very economical.
According to the data in Netherlands Published Specification No. 69/08790 excess epichlorohydrin is reacted, in a first stage, with a polyphenol in the presence of a catalyst, for example a quaternary ammonium salt to give the chlorohydrin ether, the conversion being at least 80% and preferably at least 90%, relative to the phenolic hydroxyl groups. In the second stage, an aqueous sodium hydroxide solution which contains 0.80 to 0.99 equivalent of sodium hydroxide per phenolic hydroxyl group is added, water being distilled off azeotropically. The glycidyl ether is additionally subjected to a postdehalogenation.
According to the disclosures in Netherlands Published Specification No. 70/08287 excess epichlorohydrin is reacted, in a first stage, with a polyphenol in the presence of a catalyst, for example a quaternary ammonium salt, to the chlorohydrin ether, the conversion being at least 5%, but less than 80%, relative to the phenolic hydroxyl value. In the second stage, an aqueous sodium hydroxide solution which contains 0.80 to 0.99, preferably 0.92 to 0.98, equivalent of sodium hydroxide per phenolic hydroxyl group is added, water being distilled off azeotropically whilst recycling the dehydrated epichlorohydrin. The glycidyl ether is additionally subjected to a post-dehalogenation. The quoted contents of easily saponifiable chlorine in the resulting products of the process are between 0.075 and 0.20% by weight.
U.S. Pat. No. 2,848,435 describes a process for making glycidyl ethers of polyhydric phenols (bisphenol A) which uses isopropanol as a cosolvent along with liquid caustic or solid caustic pellets. Solid caustic was used without substantial water addition. Even when a 9:1 molar ratio of epichlorohydrin to bisphenol A was used, the resulting liquid resin had a low epoxy value.
U.S. Pat. No. 2,995,583 describes a process for making glycidyl ethers of polyhydric phenols including novolac resins which uses a concentrated aqueous solution of an alkali metal hydroxide as the alkali source.
U.S. Pat. Nos. 3,766,221 and 3,980,679 describe processes for preparing glycidyl ethers wherein the etherification step is first carried out in the presence of a catalyst specific for the formation of the chlorohydrin intermediate, such as choline, a choline salt or a quaternary ammonium salt in the absence of alkali, followed by the addition of solid alkali metal hydroxide to effect the dehydrochlorination reaction to the desired epoxy compound.
The instant process is an improvement over the processes of U.S. Pat. Nos. 3,766,221 and 3,980,679 in that no catalyst specific for the formation of the chlorohydrin intermediate is needed, the etherification and dehydrochlorination are carried out in a one step process in the presence of an alcohol as cosolvent, and where the solid alkali metal hydroxide is used in the form of tiny beads.